We have developed a non-invasive imaging method that can quantitatively measure the extracellular pH (pHe) in tumors. Our MRI method uses Chemical Exchange Saturation Transfer (CEST) MRI to assess tumor acidosis, termed acidoCEST MRI. We have recently translated acidoCEST MRI to a clinical 3T MRI scanner, we have tested volunteers with our protocol, we have an approved IRB protocol for our first clinical trial, and we have assembled an exceptionally strong team of investigators to perform our clinical studies. Therefore, all steps are in place to start our clinical trials. For Specific im 1, we propose to correlate tumor pHe in breast cancer patients with tumor subtype and ex vivo correlates of acidosis, hypoxia, and glycolysis. Our rationale is based on our extensive results from our pre-clinical studies that have shown how more aggressive tumor models have a higher tumor acidosis. We have also shown that tumor pHe is correlated with high expression of carbonic anhydrase IX, which is an established biomarker of tumor hypoxia. We will perform acidoCEST MRI scans with breast cancer patients, and obtain ex vivo correlates from resected tissues during their standard care. For Specific Aim 2, we propose to use acidoCEST MRI to monitor the early response to neoadjuvant chemotherapy in breast cancer patients. Our rationale is based on our pre-clinical imaging results that have shown how Everolimus (an mTOR inhibitor) raises tumor pHe in a lymphoma model within 1 day of initiating therapy, and then returns to an acidic state after 7 days of daily treatment, which foreshadowed the temporary tumor growth delay of this drug treatment. We will perform acidoCEST MRI scans before and 4 weeks after initiating neoadjuvant chemotherapy with dose dense doxorubicin-cyclophosphamide followed by paclitaxel. We will correlate initial tumor pHe and the change in tumor pHe with clinical outcome. Based on these results, our DELIVERABLE is an innovative, noninvasive imaging method that can generate parametric maps of tumor pHe, which can be used to improve diagnosis and evaluate early response to chemotherapy for breast cancer patients.